This invention relates to vehicular mirrors and, more particularly, to rearview mirrors for vehicles which incorporate a thin layer of an elemental semiconductor to provide luminous reflectance levels adapted to reduce glare while maintaining visibility.
Vehicular rearview mirrors, especially for the exterior of an automobile or truck, are broadly classified as either spectrally nonselective, i.e., achromatic, metallic or silvery in appearance, or spectrally selective, i.e., those which use light interference to enhance reflectance in some portion of the visible wavelength spectrum relative to other portions. For example, a commonly available first surface, chromium coated glass mirror is a spectrally nonselective or metallic appearing mirror. Commercially available blue mirrors which enhance reflection in the blue region of the visible spectrum are exemplary of spectrally selective mirrors.
It is desirable in both spectrally nonselective and selective vehicular mirrors to reduce glare and provide an antidazzling effect while maintaining sufficient luminous reflectance to provide a proper image. Such an image is bright enough that the driver can quickly, accurately and easily gather information about the environment even in low light level conditions, but not so bright as to act as a source of glare from following headlights at night. Luminous reflectance of rearview mirrors is measured by using a light source which models that from a headlight and by using a detector with a filter which mimics the spectral selectivity of the human eye in its day adapted (photopic) mode. Measurements of luminous reflectance are performed in accordance with SAE (Society of Automotive Engineers) Recommended Practice J964 for measurement of rearview mirror reflectivity. In the United States, governmental regulations such as Federal Motor Safety Standard 111 require a minimum mirror luminous reflectance of at least 35%. In Europe, European Economic Community Council Directive 71/127/EEC requires a similar minimum luminous reflectance of at least 40% for vehicular mirrors. On the other hand, a maximum luminous reflectance of 60% to 65% has been found acceptable for glare reducing rearview mirrors as compared to the luminous reflectance of a full reflectivity mirror of about 80% to 90% of the incident light.
In addition, spectrally selective mirrors may be used to further optimize human sight in low light level or night conditions. As indicated above, luminous mirror reflectance depends both on the type of light source projecting light on a mirror as well as the type of detector which senses the reflected light. The human eye is a detector which adapts to various levels of ambient light by changing its sensitivity to various colors. During the day when light is abundant, human eye sensitivity is highest in the green spectral regions. As light level drops, however, the peak eye sensitivity moves toward the shorter, blue wavelengths. Since headlights have a spectral emission that is strong in longer yellow wavelengths but weaker in blue, a glare-reducing or antidazzling mirror which optimizes low light vision should accentuate reflectance in the blue regions [400 to 500 nm. wavelengths or thereabouts] where the eye is most sensitive but reduce reflectance in the yellow regions [above 560 nm. wavelengths or thereabouts] thereby reducing headlight reflectance. Such a mirror is, therefore, spectrally selective and blue in color.
In the past, both spectrally selective and spectrally nonselective vehicular mirrors have employed coatings of metal, dielectric materials or combinations thereof on glass or other substrates. While such metal/dielectric layers have functioned adequately, various embodiments have been expensive to manufacture due to the necessary coating, cutting, bending and heating procedures. Moreover, in many prior known mirrors, substantial thicknesses of metal or dielectric coatings have been required to provide optical thicknesses necessary for proper reflectance or spectral selectivity. Increased thicknesses require additional material and add expense to production costs.
Also, many vehicle manufacturers specify first surface mirrors on the vehicle exterior in order to reduce ghost or secondary reflections and images. In such mirrors, the reflective coatings are exposed to the elements and can degrade more quickly than second surface mirrors.
Moreover, the manufacturing processes necessary to make prior known spectrally selective and nonselective mirrors have often required costly, time consuming procedures which require heating and bending of glass prior to applying any coating. As is well known, the coating of a curved substrate with a uniform thickness thin layer is more difficult, time consuming and expensive than coating flat glass because of special fixturing required and the difficulties of cleaning curved surfaces, for example. Again, such procedures increase production costs.
Further, the production of prior known spectrally selective and nonselective mirrors has often required greatly differing combinations of layers and materials. The use of one or a few types of layers to produce spectrally selective as well as nonselective vehicular mirrors was difficult. Hence, modifying production techniques to incorporate the varying types of materials and to switch between the differing materials at different times reduced production efficiency and added to costs.
Thus, the need has remained for a commercially acceptable vehicular mirror which can be economically produced from a material which provides both spectrally selective and nonselective mirrors, allows use on both first and second surface mirrors, and provides luminous reflectances meeting worldwide minimum safety standards while maintaining desired glare reduction.